1. Field of the Invention
The present invention relates to a process for producing a solid electrolytic capacitor, and more specifically to a process for producing a solid electrolytic capacitor using polyaniline as the solid electrolyte.
2. Description of the Prior Art
A solid electrolytic capacitor using a high-polymer, such as polyaniline, as the solid electrolyte has a lower impedance in a high-frequency region compared with a solid electrolytic capacitor using an inorganic electrolyte, such as manganese dioxide, as the solid electrolyte. And now, a solid electrolytic capacitor using polypyrrole as the solid electrolyte is commercially manufactured.
There have been proposed as conventional processes for producing solid electrolytic capacitors using conducting polymers such as polyaniline as solid electrolytes, a method in which polyaniline or polypyrrole is deposited onto a surface-oxidized film by a monomer such as aniline or pyrrole on the surface-oxidized film, and a method in which a fused conducting polymer such as polyaniline or polypyrrole or the solution thereof is deposited onto a surface-oxidized film of a film-forming metal or valve metal such as tantalum and aluminium.
For example, Japanese Unexamined Patent Publication (Kokai) No. 64-24410 discloses a method in which a conducting polymer layer is formed by sequentially introducing a monomer and an oxidant onto a surface-oxidized film and allowing them to react on the surface of the oxidized film. In this publication, a method for producing a solid electrolytic capacitor by forming a solid electrolyte layer consisting of a dielectric polymer compound on a dielectric oxidized film, in which the monomer of a dielectric polymer compound in a liquid state is introduced onto a dielectric oxidized film (Step 201 in FIG. 3), and then an oxidant solution containing an organic acid or the salt thereof is introduced (Step 202 in FIG. 3).
In Japanese Unexamined Patent Publication (Kokai) No. 6-271655, there is disclosed a method for forming a conducting polymer film on the entire surface of a porous electrode by impregnating a solution that contains a monomer forming a conducting polymer and a carrier electrolyte into a porous electrode having minute pores, such as a sintered tantalum electrode, and electrolytically oxidizing it in the carrier electrolyte.
There are some problems when polyaniline is to be formed on the surface-oxidized film of a film-forming porous metal using conventional methods described above.
The first problem is that since a thick polyaniline layer is difficult to form on the surface of the porous metal, the process for forming polyaniline layers must be repeated resulting in poor productivity.
In the process for producing a solid electrolytic capacitor, a solid electrolyte is deposited on the surface-oxidized film of a film-forming porous metal, and then an electrode is tapped using carbon paste or silver paste and the porous metal is encapsulated using a resin material. In order not to damage the oxidized film during these processes, a solid electrolyte layer of a thickness more than a certain value (5 .mu.m or more) must have been formed on the outer surface of the porous metal.
In conventional processes, however, since the thickness of a polyaniline layer formed in a cycle of a series of steps is small, and a number of steps must be repeated in order to achieve a desired thickness.
For example, in the description of an embodiment of the above Japanese Unexamined Patent Publication (Kokai) No. 64-24410, although a method for depositing polyaniline on a porous metal by introducing into an aniline monomer the porous metal, and thereafter introducing a mixed solution of an oxidant and a protonic acid (see FIG. 3), when this method is actually carried out, the thickness of a polyaniline layer formed in a cycle of a series of steps was 1 .mu.m or less.
Thus, in conventional methods, the thickness of a polyaniline layer formed in a cycle of a series of steps was thin and limited. In order to form a multilayer structure to achieve a sufficient film thickness, therefore, a number of cycles had to be repeated resulting in poor productivity.
The second problem is that polyaniline is difficult to form on the edges of a porous metal (for example, edges and vertices if the porous metal is a rectangular parallelepiped), and defective capacitors due to the damage of oxidized films on the edges of the porous metal are frequently produced.
In order not to damage the oxidized film during steps such as encapsulation by exterior resin after the formation of the solid electrolyte described above, the solid electrolyte must also be deposited on the edges of the porous metal.
In conventional methods, however, polyaniline was difficult to form on the edges of the porous metal.
That is, in conventional methods, although desired thickness of polyaniline layers on the flat surface of the porous metal could be achieved by repeating steps for depositing polyaniline layers, little polyaniline was deposited on the edges of the porous metal. For example, in the description of an embodiment of the above Japanese Unexamined Patent Publication (Kokai) No. 64-24410, the thickness of polyaniline deposited on the edges of the porous metal was 0.5 .mu.m or less even if a series of cycles were repeated five times.
Therefore, it is an object of the present invention to provide an efficient process for producing a polyaniline solid electrolytic capacitor.
It is another object of the present invention to provide an process for producing a polyaniline solid electrolytic capacitor of a low defective electrical properties of produced capacitors.